Indian Journal or Pure & Applied Physics Vol. 41. April 2003, pp. 295-300

Comparative dielectric study of mono, di and trihydric alcohols R J Scngwa

Microwave Research Laboratory, Dcpartmem or Ph ysics. JNY Uni versi ty, Jodhpur 342 005

Received 19 August 2002, rev ised 20 January 2003; accepted 25 M arch 2003

Measured va lues of dielectric permi tt ivit y E' and dielectric loss E" of ethyl alcohol (monohydric alcohol), eth ylene glyco l {d ihydric alcohol ) and glycero l (trihydric alcohol) over the whole frequency range or dielectric absorpti on i .. e. - I OM Hz to 20 GHz. at 25 oc have been reported. The values orE' and£" of de-ionized water have also been reported in the same frequency range. The dielectri c dispersion behaviour or ethylene glycol obey the Co le-Co le model while for ethyl alcohol and water molecules it can he described by Debyc model. In case or glycerol, dielectric di spersion shows th e Col e­

Davidson type dielectric di spersion. The stati c dielectri c constant £.,, high frequency limiting dielectri c constant £ . . average relaxation time 1.,. dielectric distri bu ti on parameter a or p, free energy or activation t:J.F, and Kirkwood co rre lation fac tor g

have been cktcnnincd for these hydrogen-bonded liqu ids. The values or£,, and 1., are interpreted in terms or number of carbon atoms and the number or hydroxyl group/s present in the molecu lar structure. The effect or viscosit y and imermolecular association on the average relaxation time or these molecules has been recognized. Intramolecular group rotations clue to break ing and reforming or hydrogen bonds have also been discussed by considering the observed va lues or £~ -n l/ {n iJ, square or refractive index) and the corresponding dieleetric dispersion models or these liquids containing different nu mber or hydroxyl groups in their molecular structu res .

[Keywords: Dielectrics . Ethyl alcohol. Ehty lcne glycol , Glycero l, Trihyclri c alcohol!

1 lntJ·oduction

Di ~ l ectric relaxation of a hyd rogen-bonded molec ul e depends upon the molecular size, shape, in tra- and intermolecu lar interact ions and can be used to in ves tigate molecuiar and intramolecul ar motions and thei r relati on to molecul ar structure 1

-1' .

These studi es can be used to evaluate the barrier height hinder ing internal rotation s, prob lem of compkx fo rmati on, dipo le-dipole interacti ons and other short-ran ge intermolecular forces .

Due to hyurogen bonding, alcohols in particul ar, have been th~ subject of frequ ency dependent Jiclectric me:,sun;~ rnents in their pure liquid state and also in dilute sol uti on of non-po lar so lvents since the init i:ll stagc of relaxa ti on spectroscopy. Their long relaxation times did all ow for inrormative -;t udi ·:~. Tec hnica l dcvc l opm~nts

facilit:ttcd tnc.tsu rcmcnts unly ovcr :t limi ted r:mge ,,r ,-L'I.ll ivc ly lnw frL·qucncic" (S 100 MHz), while, nu\\ -<~ - days. !'road banJ relaxation spectra are <lL'l't:~~ ih lc, n1, cri ng the whole dielectric absorption r,!Pg-:. For pn mary al cohol s, the correlated motion or tht.. 111Llicc,des i ~ respons ible for the pt incipal rc 1ax atiun contribution and their dielectric comp lex plane plots an~ often round to be close to the Debye

type. It is interes ting to co mpare the di elec tri c behaviour of monohyd ric, dihyd ri c and trihydric alcohols studied by using single measurement advance techn ique for the ir molecular conformations . Conformational changes originating in monohydri c alcohols as a consequence of their interacti ons with dihydri c and trih ydric alcohols in binary mixtures ha ve aroused considerabl e interes t7 - ~. Dielectric re la xati on srudi es of binary mixtures of primary al cohols and pol yhydri c al cohols, have been undertaken under va rying conditions of co mpo,- iti ons and tempera ture, to help in fo rmul ating adequa te models or liquid re laxation and also in obtai ning information about the relaxati on processes in hydrogen-bonded mixtures . Many such studies of pr imary and po lyhydric alcoho ls in aqueous so luti ons are pres•:ntly in hand 1

" -1', to ga in some insight into the change in

homogeneous nature or lll(llecular int eracti ons bet v-,:cen the alcoho ls ~tnd water ancl their dy namics. over whole concentration rangL' ul the binary mixtures .

The objective o r the pres~ nt paper i~. to repo rt the broad band dielectr ic d<tla or eth yl alco hol (monohydri c alcohol ), l-' thylcnc glyco l (dihydric alcohol) and glycerol llrihyclric ;tlcohoi). over the

296 INDIAN J PURE & APPL PHYS, VOL 41 , APRIL 2003

Table I -Values of viscosity 11 . static dielectric constant£,, high frequency dielectric constant£~ . square of refrac tive index n 02

,

L. -n1} , average relaxation time 1,, distribution parameter a or ~- free energy of activation D.Ft. dipole moment u and Kirk wood correlati on factor g or different alcohols and water at 25 oc ll (m Pas)

a D.Ft (kcal mol" 1

)

Gl ycerol

964.00 42.50 4.92 2.17 2.75 1247.00 0.00 0.64 5.28 17.07 2.56 2.60

Ethylene glycol

16. 30 4 1.20 4.85 2.03 2.82 92.40 0.09 1.00 3.75 13.58 2.38 2.40

Ethyl alcohol

1.12 24.40 2.35 1.85 0.50 146.20 0.00 1.00 4.02 9.03 1.73 3.12

Water

0.89 78.80 5.60 1.77 3.83 8.30

wide frequency range of die lectric absorption which were carried out by using single measurement advance techniques for their comparison and record. The evaluated die lectric parameters are used to discuss the change in the shape of dielectric dispers ion with increase in hydroxyl groups in the molecular structure of primary alcohols to the polyhydric alcohols . Further, the dielectric parameters are also used to explore the molecular dynamic of intermolecular H-bonded species and the hindrance to intramolecular rotations in these alcohols.

2 Experimental Details

The permittivity E' and dielectric loss E" of ethylene g lycol 6 and double-di stilled de-ionized water 14 in pure liquid state were measured, using an HP 85 I OC vector network analyzer with the he lp of an HP8507 B dielectric probe kit , in the frequency range I 00 to 20 GHz. The values of E' and E" of ethyl alcohol were performed using the time domain reflectometry (TDR) in the frequency range I 0 MHz to 20 GHz. For glycerol, the values of E' and E"

were measured by the impedance-material analyser (HP 4291 A) in the frequency range I MHz- I .8 GHz and by the TDR in the frequency range of l-25 GHz. Viscosity of these liquids were determined using Ostwal viscometer. The refractive index n0

was measured by Abbe· s refractometer. All these measurements were made at 25°C. Glycerol, ethylene g lycol and ethyl alcohol of AR grade were used for the ir dielectric measurements .

0.00 1.00 2.33 9.82 1.82 2. t)(i

3 Data Analysis

Frequency dependence curves fo r the dielectric permittivity (E') and absorption (E") of ethyl alcohol (CH,CH20H), ethylene g lycol (HOCH2CH 20H), glycerol (HOCH2CHOHCH20H), and water (H20) are plotted in Fig. I at 25 oc. To evaluate various dielectric parameters, the frequency dependent experimental complex permittivity data were fitted by the non-linear least-squares method to the Havriliak-Negami express ion 15

:

E -E £*(co)= r'- j£" = £~ + " a

[1 +(jwr")H1]'

3 ... ( I)

with £,., E~ , '!., , a and ~ as fitting parameters. The values of dielectric parameters £,, £~ , -r,., a and ~ obtained from fitting into the Eq. ( I) for these molecules are recorded in Table I. Eq. (I) includes the Cole-Cole 1r' (~= 1 ), Cole-Davidson 17 (a=O) and Debye 1x (a=O, ~=I) relaxation mode ls. The values of free energy of activation , t,.Ft have been evaluated by treating dielectric rel axation as rate process, using Eyring's rate equation. The evaluated values of /j.Ft are also given in Tab le I . The va lue of g!l2

, i.e. the product of the Kirkwood correlation factor , g and the square of the molecular dipole moment, ll have been calculated fort ese molecules at 25 oc using the Kirkwood equation 1 ~. The g value has been evaluated using the available literature value of molecular dipole moment determined in dilute solutions20

·2 1

. These values are given in Table

I. The viscosity YJ , and square of refractive index n02

SENGW A: DIELECTRIC STUDY OF ALCOHOLS 297

0·01 f ( GHz)

Fig. 1 _ Frequency dependence of the dielectric permittivity£' and loss£" at 25 oe: (0 ) Water; (• ) Glycerol; (MEthylene glycol ; {0 ) Ethyl alcohol

of these molecules at 25 oc are also recorded in Table I.

4 Results and Discussion

4.1 Dielectric dispersion

Fig. I shows the variation of £' and £" for the alcohols and water with frequency . From these plots, the values of£' and £" at different frequencies can be read. In these die lectric dispersion plots, only one re laxation peak is observed for the alcohols over wide frequency range. For these molecules, it is found that, the frequency for maximum dielectric loss vary monotonously with increase in number of hydroxyl groups in the molecular structure. The trends of maximum loss frequency for these hydrogen-bonded molecules increases in the order glycerol < ethyl alcohol < ethylene glyco l ~ water. Further, the dielectric dispersion behaviOur of ethylene glycol obey the Cole-Cole dispersion while, in case of ethyl alcohol and water, the observed dielectric dispersion behaviour is a simple Debye-type. Dielectric dispersion of glycerol can

satisfactorily be represented by Cole-Davidson model. These results confirm that, as the number of hydroxyl group increases in the alcohol m?lecul ~1 r structure, the symmetrical shape of the d1electnc di spersion about the logarithm of the frequency ~f max imum loss changes into the asymmetn c relaxation curve. The Cole-Cole type dispersion behaviour of ethylene g lycol and Cole-Davidson type dispersion behaviour of g lycero l confirm that, in these molecules, besides the overall molecul ar rotation, there is a significant contribution of group rotations to the relaxation processes. The alcohols are highly water-soluble. Earlier studies22 · 2~ of these alcohols in water mixtures confirmed that, the shape of the dielectric dispers ion curves of alcohol-water mixtures are almost c lose to the di spersion shape of the individual alcohol which is mixed in water althouoh, there is strong interacti ons between

b n alcohol and water molecules. Lux & Stockhausen--studied the ethyl alcohol-water mixtures, over whole concentration range, at I I spot frequ encies ranging between 5 MHz and 72 GHz, using frequency

298 INDIAN J PURE & APPL PHYS , VOL 4 1, APRIL 2003

domain techniques and interpreted the values in Debye type di spersion. S imilarly, the complex perm1tt1 v1ty data measured by time domain spec troscopy at frequencies from 0 .0 I to I OGHz for ethylene g l yco l-water~' mixtures were fitted into Co le-Cole type di spers ion, while glycero l-water24

mi xture data were inte rpreted we ll by Co le­Davidson di spersion.

4.2 Dielectric constant

The die lectric constant of a hydrogen-bonded liquid at constant tempera ture is mainl y dependent on the fo llowing factors: the dipo le moment , the number o f molecules per unit vo lume and Kirkwood corre lati on fac tor. In ter-molecularl y hydrogen-bonded substances have hi gh va lue of

stat ic d ie lectri c constant £,, whil e, low values of £, are usua ll y found in intra-molecularl y hydrogen­bonded substances . Tabl e I shows that , the large va lue of £, for glycerol and ethylene glycol is due to the inte rmolecul ar hydrogen-bonding in pu re liquid state a lthough, these molecules have diffe rent nu mbe r of carbon atoms in the ir molecul ar struc ture. G lyce rol has three carbon atoms while, in the structure of e thy lene g lyco l, there are two carbon atoms, but , the £, va lue fo r both the liquids are nearly equal. In case of homologous series, the value of£, decreases with the increase in number of carbon atoms in the chain of the molecule. Due to presence of one more carbon atom in glycerol in comparison to ethylene glycol, the va lue of £., for glycero l should be lower than the value of e thylene glyco l. Further, in case of ethylene glyco l and ethyl

a lcoho l, the £., of ethylene g lycol is much higher

than the £, value o f ethyl a lcoho l a lthough, both have equa l number of carbon ato ms in their molecular structu re. The comparative £, va lue of glycero l, ethy lene g lyco l and ethyl alcoho l confirms that, the number of hydroxy l groups present in the molecul ar struc ture, greatly influences the ir · stati c

die lec tri c constant. F rom these £., va lues o f a lcoho ls, it is infe rred that, equal number of carbon atoms in molecul ar structure of d ifferent molecules with higher number of hydroxyl groups has high stati c d ie lectric constant. Further, the comparati ve values of £, of ethylene glyco l and glycero l confirm that, the lowering in the va lue o f £., by the increase in one carbon atom in the molecu lar structure is compensated by the addit ion of one hydroxy l group . The large va lue of £, of water mo lecu les in

comparison to the a lcoho ls is due to the large number of water molecules in per unit vo lume.

4.3 Dielectric relaxation

The large va lue o f average die lec tric re laxati on time 1., for monohydric, d ihydric and tr ih ydric a lcohol s, in their pure liquid state, is attri buted to the reorientati on of H-bonded homo-molecul ar polymeric c lu ste rs. Due to presence of hydroxy l groups 111 the molecul ar struc ture of these molecules, inte rmolecul ar assoc tated c lu sters

formed with (0 -H- ···0 ) lin kage. For the interpretati on of the re laxati on time 1 ., of these molecu les the rotational tumbling o f long-li ved c luste rs on the one hand, and the interna l f luc tu at ion or association-di ssoc iati on dynamics on the other hand, may be cons idered . T he comparat ive va lues of 1 , (T able I ) of these a lcoho ls suggest that, the length of polymeric c luste rs fo rmed in pure liqui d state vary with the number of hyd roxy l groups presen t in the a lcohol s molecul ar structure. Thi s is confirmed by the evaluated va lues of Kirkwood corre lati on factor, g of ethy lene g lyco l and e thy l a lcohol. Further, the g value o f these molecul es greater than unity confirmed that, the a lco ho l mo lecules in pu re liquid state tend to direc t themselves with para llel dipo le moment. The observed hi gh va lue of re laxation time fo r glycerol (1., = 1247 ps) in comparison to the re laxati on time of ethylene g lyco l

(1, = 92.4 ps) and e thyl a lcoho l (1,, = 146.2 ps) is due to the combined e ffec t of bigger s ize of the glycerol mo lecules and al so the high va lue of its

vi scos ity (T] =964 m pas). T he hi gh va lue of vi scos ity produces large steric hind rance to the molecul ar reorientation. If onl y, the effec t of viscos ity is considered fo r higher va lue of 1 , fo r glycero l then, the 1 , value for ethylene glyco l must be hi gher than the 1 , value of e thy l alcoho l. But. the

observed higher 1 , va lue of ethy l a lcohol in compari son to ethy lene glyco l suggests that, the length of the reorienting c lu ste r and its fl ex ibi li ty contri buted signi ficantl y, to the re laxat ion processes . Further, the strength of intermolecul ar hydrogen-bonding also affects the molecul ar dynamics. Bes ides the hi gh va lue of viscosity, low 1, va lue of ethy le ne g lycol in compa ri son to e thyl a lcoho l confirms th at, the strength of inte rmolecu lar hydrogen-bonding as confirmed by obse rved g valu e and the length of reori enta tin g po!) me rs c lu ster in ethyl a lcoho l mo lecules is much h igher than the

SENGW A: DIELECTRIC STUDY OF ALCOHOLS 299

ethylene gl yco l. Earlie r1\ it was confirmed that , the

'I., va lue of a homologous series of mono or trihydric alcoho ls increases with the increase in viscosity but, it is not true on going from mono- to di- , and di- to trihydri c alcoho ls, which impli es that, the probably different motional processes are invo lved in die lec tric re laxation and v1scous fl ow. The diffe rence in re laxati on processes of monohydric, dihydri c and trihydric a lcohols are due to presence of hydroxy l group/s in the molecul ar structure. In case of ethylene glycol, it is confirmed earlie r6 that , in addition to inte rmolecul ar assoc iation, the end hydroxy l groups a lso fo rm intra-molecul ar hydrogen bonds in dynamic equilibrium. The fo rmati on of intra-molecul ar hydrogen bonding in glycerol mo lecules is a lso ex pected due to presence of various sites avail able fo r H-bonding. Further, the glycero l molecul e promotes a set of transient cross­links between ne ighbouring molecules, through inte rmolecular H-bonding. In such a case, re levant transient structures such as, branched assoc iated species and/or chains are formed . It seems that, the cross-linked structures of glyce rol mo lecules al so contributed s ignificantl y, in the re laxati on process together with the ir hi gh viscosity and molecul ar

size. Further, low va lue of 1, o f ethylene g lyco l in compari son to the e thyl a lcoho l shows that, the probability of break ing and reforming of intermolecul ar hydrogen bonds in e thylene glycol molecul ar c luste r is fas te r due to which, the hindrance to molecul ar reorientati on decreases and hence, the 1 , of e thylene g lyco l decreases.

Tabl e I shows that, the va lues of dipo le

moment , 1-1 and Kirkwood corre lation factor, g for ethy l a lcoho l and water molecules are nearly equal

but , the 1 ., for e thy l a lcoho l molecules is very large in compari son to water molecules, however, both the systems obey the Debye di spers ion model. F rom 'I., va lues of ethyl a lcohol and water, it is inferred that, the strength of inte rmolecul ar correlation in ethyl a lcohol is much stronger than the water molecules. Further, it a lso seems that , the homogeneous c luster of e thyl a lcoho l has large number of mo lecules in co mparison to the water c luster. For water, earlie r die lec tric re laxati on studi es~5 · 2 ~> confi rmed that, six water molecules constitute a c lu ste r by intermolecul ar H-bonding. The va lue of re laxati on time (1.,=8.3 ps) for water is ass igned to the reorientati on of the water c luste r of six molecules . But, in case of ethy l a lcohol, there is

the reorientation of long c hain po lymeric c luste r and

hence, its 'I., va lue is hi gh in comparison to the 1 .,

va lue of water.

The observed va lue of di stribution parameter (a

and ~ ) for e thylene g lycol and g lycero l shows that, there is a significant contributi on of group rotati on to the re laxati on processes in these molecules. T he intramolecul ar rotation in ethy lene g lyco l and ethy l a lcoho l molecules is al so supported by the notable diffe rence between the va lue o f high f requency

die lectric constant £~ and the square o f refrac ti ve

index n02 i.e. E~-11 02 (Table I ). In these molecules,

there is intramolecul ar rotation of -OH groups abou t the C-0 bonds in dynamic equilibrium due to breaking and reforming of hyd rogen bonds in the inte rmolecular polymeric struc ture. In case of e thy l a lcohol, thi s motion may be introduced fo r the assoc iated phase which can be inte rpre ted as sw itch process of the - OH dipo le from one hydrogen bond direc tion to another. Earlie r workers~no also considered the cooperati ve switch-over mechani sm in the homogeneous inte rmolecul ar H-bonded species of e thyl a lcohol molecul es for the interpretation of the evaluated die lec tric parameters in the binary mi xtures of ethyl a lcoho l with po lar molecules. For the switc h-over mechani sm in monohydric a lcoho ls, it is poss ible that on break ing a hydrogen bond, not onl y one d ipo le switches to another pos iti on but that , a ll (ori entationall y corre lated) dipo les of a c luster (a c hain-li ke associ ated structure) a re cooperati ve ly reori ented.

For water, the significant va lue of £~ -n n" is corresponding to the second Debye d ispersion in high frequency region whic h is recentl y confirmed by advance THz refl ec ti on spect roscopy-'u2

Localized mot ions within the ne twork of H-bonded water molecules cause the high freque ncy re laxa ti on process of double-Debye mode l of water.

The observed va lue of free energy o f act ivat ion I:!.F, for these a lcoho ls are in agreement with the acti vati on energy requi red for the breakage of the hydrogen bond . Due to breaking of hydrogen bonds, there is reorientati on o f the H-bonded homo­molecular cluste rs. T hi s reorientating c luste r form new hydrogen bond with other c lu sters, in dynamic equilibrium, due to whic h there is c hange of c luste rs size of these a lcohols. During break ing o f H-bond, there is al so the orientati on o f -OH di po le about its C-0 bonds.

300 INDIAN J PURE & APPL PHYS, VOL 4!, APRIL 2003

5 Conclusion

From the comparative die lectric studies of monohydri c, dihydric and trihydric a lcohols , it is infe rred that, the static di e lectric constant of the alcohol increases with increase in number of hydroxy l groups, but a lso decreases with increase in carbon atl•ms in the molecul ar stmcture. Different homo logous series of alcohols are assoc iates to form multimers due to intermolecular hydrogen bond with parallel dipo le moment as confirmed by Kirkwood correlation factor. The comparative relaxation studies a lso confirmed that, the re laxation time of alcohols is influenced by the viscos ity, number of hydroxyl groups present in the molecular structure, and the strength of intermolecular hydrogen-bonding. Further, diffe rent types of mechani sms are responsi ble fo r reorientational motion in these H-bonded molecules due to presence of diffe rent number of sites availabl e for H-bonding.

The hig h value of re laxation time for mono hydric alcohol in comparison to the dihydric a lcoho l confirmed that, the intermolecular association masks the e ffect of viscosi ty. Further, it seems that, the length of the reorientating intermolecular pol ymeric c luste r dec reases with increase in number of hydroxy l groups in these alcohols .

Acknowledgement

Author is thankful to Prof S C Mehrotra, Department of Electronics and Computer Science, Dr BAM University, Aurangabad and Prof N Shinyashiki , Department of Phys ics, Tokai Uni vers ity, Tokai , Japan, for providing the wide frequency range va lues of E' and E" of ethy I a lcohol and glycero l, respec tive ly.

References

Shiyanshiki N. Yagihara S, Arita I & Mashimo S, .I Chem Ph1•s B, 102 ( 199X) 3249.

2 Vyas A D & Rana V A. Indian .I Pure & Appl Phys, 39 (2001) 3 16.

3 Madhurima V. Murthy V R K & Sobhanadri J, Indian .I Pure & Apf' l Ph1·s. 36 ( 199g) gs.

4 Ch itra M. Subramanyam B & Murthy V R K, In dian 1 Pure & Af'f'i Phrs . 39 (200 I ) 46 1.

:'i Scngwa R J, Chaud hary R & Mehrotra S C, Polymer. 43 (2002) 1467.

6 Scngwa R J, Kaur K & Chaudhary R, Poll'll l Int . 49 (2000) 599.

7 Wi lke G, Betting H & Stockhausen M, Phvs Che111 Liq, 36 ( 1998) 199.

8 Chaudhari A, Raju G S, Das A el a /., In dian .I Pure & Af'f'l Phys, 39 (200 I) l l:!O.

9 Khirade P W, Chaudhari A, Shinde J B el a/. , .I Sol Chem, 28 ( 1999) 1037.

10 Sudo S, Shiynashiki N & Yagihara S, 1 Mol Liq, 90 (200 1) 11 3.

II Salta U & Ghosh R, .I Phys D: Appl Ph1•s, 32 ( 1999) 820.

12 Sengwa R J, Chaudhary R & Mehrotra S C, Molcc Pftys, 99 (200 I ) 1805.

13 Lux A & Stockhausen M, Phvs Chem Li<J, 26 ( 1993) 67.

14 Sengwa R J & Kaur K, Polym lnt , 49 (2000) 13 14.

15 Havrili ak S & Negami S, Polwner 8 ( 1967) 16 1.

16 Cole K S & Cole R H, 1 Clrem Phvs, 9 ( 1941) 34 1.

17 Davidson D W & Cole R H, .I Chem Phys. I X ( 1950) 1484.

18 Debye P. Polar molecules (Chemical Catalog Co. New York). 1929.

19 Bottcher C J F. Themy of c:lectric polarization, Vo l I (Elsevier . Amsterdam), 1973 .

20 Hill N E, Vaughan WE. Pri ce A H & Davies M. Dic:lectric profJerties and molecula r hclw viour (Reinhold . London ). 1969.

21 McClell an A L, Tables o/' e.rpaimenlal difJO!e momen ts (Freeman , San Francisco), 1963.

22 Lux A & Stockhausen M, Phvs Chcm LicJ. 26 ( 19<J3) 67.

23 Shinyashiki N, Sudo S, Abe W & Yag iha ra S . .I Clrem Phrs, 109 ( 1998) 9843 .

24 Bateman J B & Gabriel C. .t Chem Soc Famdav Trans 2. 83 ( 1987) 355.

25 Mashimo S, Umehara T & Redlin H, .I Clrem Plrvs, 95 ( 199 1) 6257.

26 Mas himo S. Miura N, Umehara T. Yagihara S & Higasi K. 1 C/rem Plrys , 96 ( 1992) 635g.

27 Mandai M, Frood D G, Habibullah M, Humcniuk L &

WalkerS , 1 Chem Soc Famdar Trans I . 85 ( 19X9) 3045.

28 Minami R, Itoh K. Sato H, Takahashi H & Hi gasi K. Bull Chem Soc 1pn , 54 ( 198 1) 1320.

29 Asaka N, Shinyashiki N, Umchara T & Mash imo S, .I C/r em Phys, 93 ( 1990) 8273.

30 Schwerdtfeger S, Kohl er F, Pottel R & Kaatze LJ, .I Chon Phys , 11 5 (200 1) 4 186.

31 Ronne C. Thrane L, Astrancl P-0 et a / .. .I C'lre111 f'h1·s . I 07 (1997)5319.

32 Ronne C. Astrand P-0 & Kciding S R, Ph1·s R tT Lell . 82 ( 19')9) 28l.l8 .